Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 15 Apr 2022 at 12:45, Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Fri, Apr 15, 2022 at 12:22:27PM +0200, Ard Biesheuvel wrote:
> >
> > Subsequent objects are owned by the driver, and it is the
> > responsibility of the driver not to modify the fields while it is also
> > mapped for DMA (and we have had issues in the past where drivers
> > violated this rule). So as long as ARCH_KMALLOC_ALIGN guarantees
> > actual DMA minimum alignment for both the start and the end, we
> > shouldn't need any explicit padding at the end.
>
> I don't understand why this is guaranteed.  The driver context
> size is arbitrary so it could end in the middle of a cacheline.
> The slab allocator could well lay it out so that the next kmalloc
> object starts right after the end of the context, in which case
> they would share a cache-line.
>

If this is the case, things are already broken today. We never take
ARCH_DMA_MINALIGN into account when adding the driver ctx size to the
overall allocation size.

> The next kmalloc object could be (and in fact is likely to be)
> of the same type.
>
> Previously this wasn't possible because kmalloc guaranteed
> alignment.
>

Either it does or it doesn't. If kmalloc() guarantees the actual DMA
alignment at both ends, the situation you describe cannot occur, given
that the driver's slice of the request/TFM structure would be padded
up to actual DMA alignment, in spite of whether or not
ARCH_DMA_MINALIGN exceeds that. So it would never share a cacheline in
practice, even though they might live in the same 128 byte aligned
region on a system that has a minimum DMA alignment that is lower than
that.

If kmalloc() does not guarantee that the end of the buffer is aligned
to actual DMA alignment, things are already broken today.